After further investigation, I noticed that the drive appeared in the disk management utility on Windows XP as a "GPT protective partition", thus Win XP could not access it.
This came about via my own lack of knowledge when I initialized the new disk in Windows 7 and selected "GUID partition table (GPT) partition style".
I should have selected the master boot record (MBR) method.
I couldn't figure out how to revert the disk. A quick web search for "GPT protective partition" led me to this page: How to delete GPT Protective Partition - Paul Gu|blog
In Win XP, I followed the command prompt instructions given on that page, and re-initialized, formatted NTFS, etc. Now the drive works fine in both OS
Fortunately I had very little data to lose, but it was an exercise in wasted time due to my own ignorance.
Thanks to AddRam for some good advice as well (Partition Wizard Bootable CD) - Had I heard it before I reformatted it may have saved me some time and effort