Can I replace WD20EF R X with WD20EF P X?

Yes — in most cases you can replace a WD20EFRX (“R”) drive with a WD20EFPX (“P”) drive, but there are some caveats you should check. I’ll break down what these model codes mean, what differences might matter, and when replacement is safe (or risky).

What do “R” and “P” mean?

The designations you mention are likely Western Digital’s NAS‑/Red series drives:

WD20EFRX (ending in “R X”) is an older WD Red (or Red/Plus) NAS‑optimized drive.

WD20EFPX (ending in “P X”) is a WD Red Plus drive, meaning it is marketed as a higher/revised spec variant.

The “Plus” models are often meant to improve performance or reliability (firmware, vibration control, etc.) compared to earlier Red models.

One key issue in NAS drives is whether they use CMR (Conventional Magnetic Recording) vs SMR (Shingled Magnetic Recording). Many people avoid SMR in RAID/NAS setups because SMR can degrade performance under heavy random writes or rebuilds. On forums, users mention that the older “R” models might be SMR and that “P” models are safer (CMR) choices.

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Thus, replacing with a “P” model is often considered an upgrade or more suitable for NAS.

What to check to ensure compatibility

Even though it’s generally possible, you should verify a few things before swapping:

Parameter Should match / be compatible Why it matters

Interface / connector Both are SATA 6 Gb/s / 3.5″ form factor So they physically connect the same way.

Capacity 2 TB (or larger) If the new drive is bigger, the NAS/RAID system might only use up to the size of the smallest drive until fully replaced.

Firmware / NAS features NASware, RAID error recovery settings To ensure it works well in NAS / RAID context.

Recording technology (CMR vs SMR) Prefer CMR SMR may cause performance or rebuild issues in RAID arrays.

Drive specs (vibration tolerance, rotational speed, cache, etc.) Close or better Big differences might affect overall array balance or performance.

If all these check out, the “P” model should work. Many users on forums explicitly say: “Do NOT buy WDEFRX drives; WDEFPX is a suitable replacement.”

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In a RAID / NAS array — how replacement works

If your WD20EFRX was part of a RAID mirror (e.g. RAID 1), here’s how replacement typically goes:

Remove the failing drive (if the system allows hot-swap) or power down and replace.

Insert the new drive (WD20EFPX). The RAID controller or NAS enclosure should detect it.

The system will rebuild/synchronize data from the remaining original drive to the new one.

After rebuild, the array is restored. If both drives become “P” models (or larger), you may be able to expand the array to use the extra space, if available.

But caution: rebuilding stresses drives, so the remaining old drive may die during the process. Always backup before such operations.

One user reported exactly your scenario — replacing a WD20EFRX in a My Cloud Mirror — and the community response was supportive of swapping in a WD20EFPX, with caution and good backing up.

Western Digital

Risks & caveats

If the original “R” model is SMR and your NAS expects CMR, mixing types could lead to performance issues, especially on rebuild or heavy load.

If firmware differences are large, the NAS might balk (though this is rare for the same series).

Rebuild failure is possible if the old drive is near its end of life.

If your NAS enclosure or RAID controller has drive compatibility listing, check if the “P” variant is supported.

Conclusion

Yes — replacing WD20EFRX with WD20EFPX is usually safe and often beneficial (if the “P” is CMR, more modern, better suited for NAS). It’s considered by many users as a valid upgrade. But before doing it:

Confirm the new drive uses CMR (not SMR).

Ensure specs (interface, form factor, etc.) match.

Backup your data.

Rebuild carefully and monitor.

If you like, I can check the exact specs of your drives (based on serials or PCB) and tell you whether in your case it's safe. Do you want me to dig that detail for your particular units?