> I have been asked to visit a friend tomorrow to sort out a problem
> with an external drive his son has bought for his Mac. He can't access
> it from his family's PCs.
>
> I don't know if he bought an Apple drive or a generic drive.
>
> I know nothing about Macs but I suspect it is caused by the file system
> on the external drive being HFS Plus.


I have exactly the same problem right now. Looking at the disk in Windows XP Disk Manager shows it as formatted GPT (the new Apple format), so I suspect someone in the family has plugged it into the Mac which has formatted it (either automatiucally or after prompting).

A bit of research revealed that GPT writes a "protective MBR" at the front of the disk, to make it impossible to read (and difficult to reformat) from Windows.

Viewed from the Mac, the disk appears to have a Time Machine volume on it.

Whether the oringal pohotos on the disk (saved from a PC) are still there (but not accessible) or completely gone I don't know yet.

Microsoft knowledge base mentions that GPT disks are readable with newer versions of W7, but not from XP.

Robert