Could not find installable ISAM

I had the same error, but none of the suggestions above worked. In my case all I had to do was to change my connection string to this:

string connStr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + FilePath + ";Extended Properties='Excel 12.0;IMEX=1;'";

Note the Single Quote around the Extended Properties attribute ('Excel 12.0;IMEX=1;'). Once I added those single quotes the error disappeared!

There's no 64 bit version of the Jet OLEDB drivers, so if you are running this on a 64 bit OS you might need to target x86 in your .NET application and not Any CPU:

I was getting this issue trying to opening an xls file with a more recent provider. I fixed this issue by changing my extended properties from

Extended Properties="Excel 11.0;"

to

Extended Properties="Excel 8.0;"

I guess Excel 11 expects an xlsx style file.