In general, HDDs are slow
ER than SSDs but
PLENTY fast for TM backups. Differences are mostly about speed of drive writes. There's no mission critical pace of how fast new file creations can get onto the TM backup drive. If it takes 30 seconds or 3 minutes, that new file write is invisible to you (background task). It's just there (on the backup) if you need it at some point.
Whether you go with SSDs or HDDs, the "slow" TM backup is the very first one- the backup of
everything. Depending on how much you have on your drive, that could take many hours or even a few days. Whichever it is, just leave the Mac on and let TM complete backup #1 while you work, play, sleep. Eventually the first one is done and all subsequent ones only need to backup the very new files you've updated/created/added in the last approx. hour (unless you are a content creator at The Flash speeds and/or acquiring new files like crazy, this is very quick on HDDs).
If you heed my advice about the offsite one for the added security of at least TWO complete backups, you'll have the hours or days first backup TIMES TWO. But then, you'll have two complete backups as you wish. Store the one offsite and leave the other onsite as current TM backup drive. The latter will be backing up "fast" on the hour. The former will be falling behind depending on how many days you leave it offsite. When you swap them, the older backup may need more than a few minutes to "catch up" on the backups it has not been doing while stored offsite... but then its hourly backups will be quick thereafter.
Paying up for SSDs will probably make those backup #1s finish modestly faster than HDDs. But after that, you probably won't even notice the backups in progress and thus not notice the difference in speed.
The money you could pay for SSDs large enough to do this well could probably buy you 3X-5X more storage on HDDs. And the key benefit of TM beyond reliable backups is that ability that gives it its name... the ability to go back in time to get earlier versions of files if latter ones are lost/damaged/corrupted. Abundant storage allows more time "travel" (more file copy backups) vs. getting less storage in SSDs and putting limits on how far back in time you can go.
It doesn't matter how fast the drive type is if the former file version you need has been overwritten because you ran out of space. Buy BIG storage- I suggest at least 3X the total amount of storage on all of the Macs in your household (so they can all TM backup to the drive)- so you have plenty of space to step back in time to get the version you want/need.
Along with the speed consideration comes price differences... which usually leads to choosing LESS storage to buy more speed in SSDs. However, less TM capacity means the backup will "fill up" and then start deleting oldest versions of backed up files to create space for new versions. Your ability to go back in time will be reducing as that happens. Think of this as sort of a rolling window of time (file versions) you can get back to if you need to recover an earlier version. This is the main reason to BUY BIG (storage) so that that old version recovery window can be much larger.
The other benefit of SSD is
silence. HDDs will be noisier than SSDs. So if that matters enough, the SSD option might win on noise. However, there is also the manual TM option, where selecting "backup now" in the TM menu triggers a manual backup. So one option if you don't want automated backups and thus silence is to save the manual backup until end of day and then make that the last thing you start when you are ready to leave the desk and/or go to bed. It can backup and make HDD noise while you sleep or are away for the evening/night.
What I did was found a very quiet spot to store the drive so I can't hear it and let the automated process run on the hour. Generally, a
long USB cable can help one find the quietest possible spot to locate the backup drive.
Lastly, perhaps the economics matters. If you dual drive it and- I'm guessing- at a storage need:
For the $1500 you might put into the two cheapest 8TB SSD sticks, you could buy 110TB of those 22TB HDDs and have up to FIVE TM backups if you wanted overkill protection. Or maybe spend $600 on the TWO 22TB for local TM backup and put the rest into a
Synology NAS to give you one more local TM backup to it plus options like creating your own cloud that you fully control and that costs $0/month in subscription fees.
Synology offers RAID5-like options so that it can backup itself in a single drive failure scenario, so that will be like you have
two TM backups on that box too. A net of this would be- essentially- FIVE duplications of files:
- Master files on your Mac,
- TM onsite disc,
- TM offsite disc,
- Synology TM backup,
- Synology RAID5-like hardware backup called SHR (or you can set it up with any of the RAID options like RAID 5 or 10). I've used SHR for about 10 years now and it works great. One great benefit of using their SHR over- say- RAID 5- is that you could initially start with as little as only 2 drives in that box and then add more storage over time... and that storage could be any size (even mixed sizes). If you have some HDDs laying around, you can combine them in the Synology SHR and put them to work. Expand as needed and replace any of them whenever you like.
With each copy, the odds in losing all get substantially lower. But biggest key is #3 and regular rotation of offsite and onsite to keep the offsite copy pretty fresh.
