If you have a formula like =[OtherBook.xls]SheetName!RangeName
and Otherbook.xls resides at C:\MyFolder
the formula (when OtherBook.xls is closed) is
='C:\MyFolder\[OtherBook.xls]SheetName'!RangeName

The limit on the length of a formula is 1024 characters.
The formula above would count as 48 characters (even if OtherBook.xls
is open and it takes the shorter form when viewed in the formula bar)

If OtherBook.xls is in a directory with a very long path, has a long
workbook name, a long sheet name and/or a long range name you might hit
the limit, particularly if the formula contains multiple references to
OtherBook -
e.g. if the formula is
=IF([OtherBook.xls]SheetName!RangeName=0,"",[OtherBook.xls]SheetName!Ra
ngeName)

then the full form of the link (when Otherbook.xls closed) would only
have to be about 500 characters to break the limit.

Hope this helps.
If not, give us an example of the formulas you are trying to create
with detail of where the Otherbook.xls is located.

Bill Manville
MVP - Microsoft Excel, Oxford, England
No email replies please - respond to newsgroup