Slackware64-14.2 still running on this desktop.

I turned off the desktop and removed the power cord to replace the video
card. The MediaSonic Probox also powered down. When I plugged in the desktop
and turned it on, along with the Probox, the two single drives in the Probox
(/data2 and /data3) automatically mounted, but the 2-drive logical volume,
/dev/md0 did not mount. When I try to mount it manually I find that it
doesn't exist.

I've gone through this video card swapping several times this week with the
LV mounting, either automatically of when I do so manually.

Where do I start looking for the reason it's not now seen?

The naming is confusing. Do you have MD Raid set up ? I would expect a software Raid device to be designated md0, unless you have LVM configured in a Raid array?

I'd start with

The first command should tell you the status of the array (if any found)
In the second command output you can search for any error about those drives or why the md is not starting.

Gerard/ctrlaltra: I need to rebuild the raid1 array, the virtual groups and the virtual array. When I check /dev/sde and /dev/sdf with cfdisk they're both marked as free space rather than ext4.

Somehow, those two disks were FUBAR'd. So, I start from scratch and rebuld them because the lV is mounted as /media/backup/ and I'll have lost the previous ones. Sigh.

Thanks,

Rich

Again, it's not clear. Are /dev/sde and/dev/sdf the two components of the software Raid array? If so, they shouldn't be formatted with any file system if you're going to put Logical Volumes on top of that array. If you're using MD and LVM you would normally create the Raid array and then add that to a Volume Group. Then you would create Logical Volumes in that group and format those volumes with your file system of choice.

In terms of naming, it's likely that your Raid array will take md0, so you should name your volume group and logical volumes something else -- lvgroup, lvvol00, lvvol01, and so on.