OP:
Your original post leaves a lot "in the dark" about what you're trying to do.
A number of questions that need to be answered:
- The drive(s) that you took out of the MacPro -- are they 3.5" or 2.5"?
- Were these drives just "regular, working drives"? In other words, they are not Time Machine backups, or another kind of backup?
- The "converter" that you bought -- is it a USB3/SATA "dongle/adapter" that looks similar to this?
https://www.amazon.com/Sabrent-2-5-...478&sr=1-2-spell&keywords=sabremt+usb3+to+ssd
If that's what you're trying to use with a 3.5" drive,
it's not going to work, because the USB bus can't provide enough power to get a 3.5" drive "up-and-running" and mounted on the desktop.
For mounting 3.5" drives via USB3, you need a "USB/SATA docking station". There are many out there, here are some from amazon:
https://www.amazon.com/s?k=usb3+sata+docking+station&crid=1SYC6MJWSF64U&sprefix=usb3+sata+doki,aps,143&ref=nb_sb_ss_sc_1_14
Here's one that could probably do the job right, at low cost:
https://www.amazon.com/Sabrent-External-Docking-Station-DS-UBLK/dp/B00IKAQ538/ref=sr_1_8?crid=1SYC6MJWSF64U&keywords=usb3+sata+docking+station&qid=1554906842&s=gateway&sprefix=usb3+sata+doki,aps,143&sr=8-8
At this stage of the game, the first obstacle to overcome is to be able to get your old drives "up, running and mounted" on the desktop of your NEW Mac.
When you "get that far", come back and let us know, and more help will be forthcoming.
Hmmm... one other thing.
On the new Mac.... have you gone to finder preferences, and then checked to show "external disks" on the desktop?
