Mac OS X Will Not Mount Pegasus R4 After a Failed Physical Disk Replacement

  • Last Post 31 July 2017
Anthony Petosa posted this 29 July 2017

Recently, I had a hard disk failure in my Pegasus R4 (first generation) RAID5-configured disk array. The fourth of four drives failed. Per the Pegasus R4/R6 Compatibilty List available on the Promise Technology site, I purchased the following compatible hard disk:

Toshiba DT01ACA100, 1 TB, 6 Gb/s transfer rate, 64 MB buffer size, 7200 RPM

It replaces the original Hitachi HDS721010DLE630, 1 TB, 6 Gb/s transfer rate, 64(?) MB buffer size, 7200 RPM disk drive. Promise Technology informed me that mixing and matching drives from different manufacturers and with (potentially) different buffer sizes has no impact on the Pegasus R4's ability to function properly. Since I used a hard disk on the compability list, I felt confident that no issue would arise after replacing the failed disk drive.

Once Installed I went through the process of manually rebuidling the replacement drive. Promise Technology confirmed that I would incur no data loss, since the disk array is set up with RAID5 and that the rebuild process would populate the replacement hard drive with the RAID5 parity bit data necessary for data redundancy. With RAID5 in a 4-disk configuration, one drive can fail without the impact of data loss. After the drive failed and up this point, where I replaced it, I made no attempt to execute neither read nor write operatoins on the disk array, even though I was informed that read operations are safe.

After installing the replacement disk drive, I ran the Promise Utility application, and the dashboard reported green in every dashboard item listed under 'System Status'. Unfortunately, my Macbook Pro's OS no longer 'sees' the Pegasus R4 logical disk drive; it does not appear on the desktop. Of course, I first tried checking Finder Preferences to ensure the 'external disks' option was checked. As I suspected this was not the cause, since I have other external drives that are mounted and reported on the desktop. Next, I ran Apple's Disk Utility application, selected the 'PROMISE PEGAGUS' volume, and clicked on the 'Mount' button at the top of the Disk Utilitiy application window. Disk Utility reported the following error:

"Mount failed: The disk 'PROMISE PEGASUS' could not be mounted. Try running First Aid on the disk and then retry mounting."

At this point I am hesitant to proceed without some reassurances. Will running 'First Aid' in any way cause data loss on my RAID5 disk array? Are there any safe 'First Aid' operations I can perform without jeopardizing my existing data?

Here are specs on my MacbookPro:

  • Retina, Mid 2012
  • 2.7 GHz Intel Core i7
  • 16 GB 1600 MHz DDR3
  • NVIDIA GeForce GT 650M 1024 MB
  • OS X Yosemite (Version 10.10.5)

Your feedback is greatly appreciated.

Thank you.

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Pradeep Chidambaranath posted this 29 July 2017

 Hi Anthony,

Kindly go to to create a support ticket with the complete subsystem for further investigation.

Go to the below link go download the latest version of the utility.  


Follow the below step to save the subsystem report for further investigation.

- Open Promise Utility.
- Click subsystem information icon on the top of the window.
- Click the lock symbol on the left bottom corner of the screen to unlock the utility.
- Click on save service report button to save the subsystem report.  


Pradeep C

Anthony Petosa posted this 31 July 2017


Thank you for the reply and your recommendations. I will create a support ticket and also download the latest utility version. However, before doing so, is there any particular reason why I would need to install the utility update? After all, the disk array was functioning properly prior to the hard disk failure, and I replaced the failed disk with one enumerated in the compatibility list. What could have changed that would require a utility update?

I'm trying to understand the problem before I commit to invasive measures like software and/or firmware updates.

Thank you.