[PeterT]

Originally Posted by Backi

Oh, I see. For NTFS it is allowed.
Is there any possibility in Calibre to determine the target file system?

Even though it might be a valid NTFS name it would still cause issues at the command prompt level in Windows.

Spoiler:

D:\Downloads\testplus>dir > a+b

D:\Downloads\testplus>dir
 Volume in drive D is D Drive
 Volume Serial Number is 122D-9E9C

 Directory of D:\Downloads\testplus

11/03/2012  09:13 AM    <DIR>          .
11/03/2012  09:13 AM    <DIR>          ..
11/03/2012  09:13 AM               337 a+b
               1 File(s)            337 bytes
               2 Dir(s)  318,907,011,072 bytes free

D:\Downloads\testplus>copy a+b c
The system cannot find the file specified.

D:\Downloads\testplus>copy 'a+b' c
The system cannot find the file specified.

D:\Downloads\testplus>copy "a+b" c
        1 file(s) copied.

D:\Downloads\testplus>dir /x
 Volume in drive D is D Drive
 Volume Serial Number is 122D-9E9C

 Directory of D:\Downloads\testplus

11/03/2012  09:13 AM    <DIR>                       .
11/03/2012  09:13 AM    <DIR>                       ..
11/03/2012  09:13 AM               337 A_B~1        a+b
11/03/2012  09:13 AM               337              c
               2 File(s)            674 bytes
               2 Dir(s)  318,907,011,072 bytes free