Not new to Linux, but have not worked in any detailed way w/ext or lvm.

Disk (RHEL4) has some mechanical issues, and most likely has experienced a head crash. It's a very old 40Mb external (USB), and I would like to retrieve files/dirs from this disk.

What's the procedure to take this disk and mount it on a new RHEL/CentOS 5.10 or 6.3 system.

I assume it will require some use of lvm commands such as vgscan, etc.

Thanks.

40 MB or 40 GB?? I can't imagine a disk drive from the "40 MB" era still being operational, much less having a USB interface.

The first step in rescuing data from a failing disk should be to use ddrescue to recover as much data as possible to a file or disk partition on the new system. Then, depending on what could be recovered, we can take a look at what needs to be done to access the data. You should not try to play directly with the failing drive. Damage from a head crash can expand rapidly. ddrescue will make a quick pass to recover those sectors that can be read easily, and then go back to work on the troublesome ones.

Mounting it should be easy enough.

Two steps generally. One is can it be mounted and filesystem accessed to read files? If not then can a program like testdisk or some form of unstoppable copier be used?

I'd think it unlikely that you have a lvm setup but maybe. Almost any live cd/dvd/usb could be used I'd think to try to recover files.

Depending on how valuable versus how bad it is you may wish to try to dd (or special dd that ignores errors) the drive to an image file and then work on just the image file. As rknichols suggested.