New-MailboxExportRequest Error details: Header file length is zero

Hi,

I am using this cmd on our mailbox database to export all mailboxes to pst;

foreach ($i in (Get-Mailbox -ResultSize Unlimited)) { New-MailboxExportRequest -Mailbox $i -FilePath "\\servername\pst \$($i.Alias).pst" }.

First time it runs fine, but the second time it fails on all mailboxen bigger then 1 gb in size with the following error :

Unable to open PST file 'servername\pst\failedusers.pst'. Error details: Header file length is zero, is the
file is from a previously failed pst export, please delete the file and resume the export.
    + CategoryInfo          : NotSpecified: (0:Int32) [New-MailboxExportRequest], RemotePermanentException
    + FullyQualifiedErrorId : 97E980C,Microsoft.Exchange.Management.RecipientTasks.NewMailboxExportRequest

All the other mailboxes below 1 gb are running fine the second time i am running the cmd.
If i remove the failed pst before running the cmd again it runs fine as well and then it creates a new pst for the mailboxes.

The Pst files are located at a nfts formatted external drive..
Anyone have a idea why its failing?

Thx in advance..