I have a hard drive from an old centos computer, and I'm trying to mount it as an external hard drive to get the data, but it's not showing up as a normal device. I have it hooked up to a usb/sata interface that has worked with other drives.

On my debian machine, one partition shows up on the desktop, and when I click on it I'm prompted to mount /dev/loop3 as an administrator. However, once inside this drive I can't find the home folder, and the file system looks only vaguely like a linux system.

On my gentoo machine, no new device appears in /dev when this drive is plugged in. I believe that this drive has a gpt partition.

Any ideas?

What version of CentOS? Is it using LVM?

With the drive connected to the debian machine post the output of the lsblk command.

It's centos 7. Not sure about LVM. Here's the output of lsblk

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
loop0 7:0 0 114.9M 1 loop /snap/core/14056
loop1 7:1 0 115M 0 loop /snap/core/13886
loop2 7:2 0 373.8M 1 loop /snap/anbox/186
loop3 7:3 0 310.8M 1 loop
nvme0n1 259:0 0 119.2G 0 disk
├─nvme0n1p1 259:1 0 512M 0 part /boot/efi
├─nvme0n1p2 259:2 0 114.8G 0 part /
└─nvme0n1p3 259:3 0 4G 0 part [SWAP]

If the drive is connected then it isn't being recognized. I would expect it to be assigned /dev/sda device ID. A desktop SATA drive would require external power separate from the USB adapter. I am not sure what loop3 is all about. Check the output of the dmesg command to look for the drive and loop3 messages

CentOS 7 would use LVM by default unless you selected another partitioning scheme.

Hmm, there's nothing in dmesg that seems linked to this drive.

Indeed, I have it in an enclosure that provides extra power. I've been able to mount other 3.5" drives using this method, although I guess it's not impossible that this drive needs even more power that it's not getting.

Does the output of the lsusb command show the usb adapter.
Do you see anything in the output of dmesg pertaining to the adapter?

Okay, it looks like the debian system wasn't actually detecting the drive after all. The first sign was when lsusb returned exactly the same output with/without the drive plugged in. When I unplugged the drive and clicked on the icon to open it, it still opened. I am actually not sure what I'm opening. It's reporting as a 318MB drive, with a file system that looks kind of like a root file system. This particular debian machine is a surface pro 4, which I had to monkey with quite a lot to get debian installed properly, so it's possible that this is some relic of a failed install.

Anyway, the short story is that nether computer seems to detect this drive. That really seems to imply that it's a hardware problem, like not getting enough power or the like. That is kind of weird since other drives have worked with this enclosure. Any other thoughts?

Usually the bridge i.e the actual USB to SATA adapter is powered via USB and the external power is for the drive itself. If lsusb shows nothing then it probably is an enclosure problem.

Yup, I just tried a different enclosure, and it works fine. Thanks for helping me figure that out!