I will answer your questions divided into parts, as you wrote.
- Is it ok to just copy the folders in the Windows XP Explorer to the new partition?
Make explorer view
hidden files, to see your files. If some apps are opened , close them before transferring your files (otherwise there can cause an error while moving a file, which is opened by an app). But you can't move all files just dragging and dropping in explorer (see the next part of the answer).
- Could anything be lost this way (hidden folders, metadata, ..)?
Yes. Here are files, which will not be moved:
1)Files, which are
in usage or
locked (are opened by an app/service/system...)
2)Files, which you can't access (because
you haven't privileges to them)
3)Windows Registry* files are locked and you can't move them (while you are in logged in).
some program settings and all other
information about files stored in Registry (which are not inside files) will be lost too.
*Windows Registry is stored in some files that are inside folder C:\windows\system32\config (the files sam,security,software, ...) and in NTUser.dat, which is in user profile's folder.
So if you want to copy ALL files manually (including that files which you can't move, while logged in), you can
use bootable CD/DVD/... .
Because then you'll have no restrictions to move your data.
For example, boot from MS-DOS with ntfs4dos or Paragon NTFS driver (if your partition is NTFS),
or use Windows Live CD/DVDs.
- What is the best way to clone a data partition in Windows XP?
A very good tool for you: Acronis® Migrate Easy 7.0 . Or use tools like it.