Cannot write to external hard drives

I have two external hard drives connected to my PC, one a LaCie and one a Pleiades. I never had any problem reading and writing to and from these drives with earlier versions of Windows, or indeed when they are connected to other computers, but Windows 7 will not let me write to them or modify them in any way, though I can still read files on them.

If I try to, e.g., create a new folder or copy a file onto the external disk, I get the message "You need to confirm this operation". If I then click on "Continue", the same message is repeated ad nauseam. I am not given any opportunity to confirm. I am the administrator and indeed only user of this computer, and this has become enough of a major problem that I am seriously considering dumping Windows 7 and going back to an earlier version.

Any help or suggestion would be very welcome.

What is the file format on those drives? Are you trying to write NTFS to a FAT32 drive? Also check permissions to ensure you have write privileges.