Cannot decrypt data on external harddisk

I used my external harddisk on a laptop, which had Symantec EndPoint Encryption (SEE) installed on it, in the past. I don't have that laptop anymore, but now I want to use the same external harddisk on another laptop without SEE installed and I'm unable to decrypt the data!

When I use the 'Removable Storage Access Utility' and choose 'Decrypt to Location', I'll receive the message "A certificate is needed for decryption. If you have a certificate on a Smart Card, please insert the card and then click OK. 

I don't have a certificate.

Could somebody please help me decrypt all those files, so I can open them again?

Thanks!